As is well known, exploration for and recovery of oil and gas has long since extended into offshore venues. Early offshore drilling operations were concentrated in relatively shallow waters. However, the number of shallow water drilling sites is finite, while the world's appetite for oil and gas is seemingly unlimited. It has therefore become necessary to conduct offshore drilling operations in waters as deep as 10,000 feet or more.
Offshore drilling operations are frequently conducted from floating platforms known as mobile offshore drilling units (MODUs). While the mooring of offshore drilling units in shallow water is relatively straightforward, the successful mooring of MODUs in deeper water can be problematic.
The traditional method of mooring MODUs in deeper water involves the use of drag embedment anchors and mooring lines which are stored on the MODU, and which are deployed from the MODU using anchor handling vessels. Some of the latest generation MODUs can carry adequate lengths of wire and chain on board, and are equipped with combination wire/chain mooring winches to moor at maximum depths of 5,000 feet of water. Large anchor handling vessels are capable of deploying and recovering such mooring legs and anchors. In even deeper water, however, the amount of wire and chain that would have to be carried on the MODU becomes too large, and even large anchor handling vessels would have difficulty deploying and recovering such mooring systems in the traditional manner.
Older generation MODUs typically cannot carry enough mooring line to moor in water deeper than about 2,000 to 3,000 feet. This water depth limit can be extended by inserting sections of wire in each mooring leg, or by pre-installing mooring legs prior to arrival of the MODU at location. Both types of extended water depth mooring legs (insert or preset) typically use modern high holding power drag embedment anchors. Large anchor handling vessels are used to install the wire inserts during mooring leg deployment or to pre-install the preset mooring legs.
One drawback to deep water MODU moorings using drag embedment anchors is that such anchors typically cannot handle uplift (vertical load), which requires both that the mooring leg is very long, and that the anchor is set very far from the MODU. In water depths over 6,000 feet the horizontal distance to the anchors can become a problem, since it could be as large as 12,000 feet or 2 nautical miles, and each mooring leg could be as long as 15,000 feet or 2.5 nautical miles. This requires an anchor spread diameter of about 4 nautical miles.
If an anchor system can be used which can handle substantial uplift or vertical load, the anchor radius and mooring line length can be reduced significantly. Driven anchor piles are capable of handling uplift, but cannot be installed in deep water nor are they recoverable. For this reason, driven anchor piles have never been used for MODU moorings.
Mooring systems employing anchors other than drag embedment anchors and driven piles have been proposed heretofore. For example, U.S. Pat. No. 4,318,641, granted to Hogervorst on Mar. 8, 1982, discloses mooring systems employing suction embedment anchors, which are capable of taking significant uplift or vertical load. However, prior to the present invention, there has not been provided a successful system for installing and recovering suction anchors in very deep water thereby facilitating ultra deep water drilling operations.
The present invention comprises a system for mooring mobile offshore drilling units which overcomes the foregoing and other difficulties long since associated with the prior art. In accordance with the broader aspects of the invention, mobile offshore drilling units are moored using anchors which are recoverable and reusable upon the completion of drilling operations. In accordance with a preferred embodiment of the invention, recoverable suction anchors are employed, and in accordance with another embodiment of the invention vertically loaded anchors are employed. Recoverable drag embedment anchors can also be used in the practice of the invention, if desired.
Regardless of the anchor type employed, the present invention comprises a method of mooring MODUs wherein a first set of recoverable preset mooring legs with suitable anchors are pre-installed at spaced apart locations surrounding an offshore drilling venue. After the recoverable preset mooring legs are installed, a MODU is positioned at the drilling venue, and connected to the preset mooring legs by short lengths of the mooring chain or wire extending from mooring winches on the MODU.
While drilling operations are being conducted from the MODU, a second set of mooring legs is preset at spaced apart locations around a second drilling venue. When drilling operations have been completed at the first drilling venue, the mobile offshore drilling unit is moved from the first drilling venue to the second drilling venue, and is secured in place at the second drilling venue by connection to the second set of preset mooring legs surrounding the second drilling venue. The mobile offshore drilling unit is then used to conduct drilling operations at the second drilling venue in the usual manner.
While drilling operations are being conducted at the second drilling venue, the first set of mooring legs is recovered from the first drilling venue and is moved to a third drilling venue. The mooring legs from the first drilling venue are then preset at spaced apart locations surrounding the third drilling venue with the installation thereof being completed prior to the movement of the mobile offshore drilling unit from the second drilling venue to the third drilling venue. This process continues until drilling operations have been completed at all of the drilling venues within a particular area, whereupon all of the mooring legs and the MODU are removed to a different area.
The present invention may also be practiced using less than two complete sets of mooring legs. In such instances, a reduced number of mooring legs, for example, one half of a complete set, is installed at a second drilling venue. The MODU is then moored to the second drilling venue and secured in place using the preset mooring legs then in place. Next, the remainder of the mooring legs comprising a complete set are installed, whereby the MODU is fully secured. This procedure is repeated until drilling operations are completed at a particular location.